The present application relates to an apparatus for converting AC voltage to a variable DC voltage for exciting the rotating field of a synchronous generator.
It has been common practice to use a full thyristor bridge circuit to convert AC voltage to a variable DC voltage for exciting the rotating field of a synchronous generator. A typical example of such a full thyristor bridge circuit is shown in FIG. 2. The full thyristor bridge circuit 100 includes six thyristors 101-106 that are connected with an AC power supply 125. A de-excitation circuit 127 is provided at the output of the thyristors 101-106 and is controlled by either a field discharge circuit breaker or an AC or DC breaker with a thyristor discharge device to reliably discharge the generator field 130.
The full thyristor bridge circuit 100 shown in FIG. 2 includes many functional advantages such as the ability to transiently invert field voltage to rapidly decrease generator field flux linkages. Additionally, the full thyristor bridge circuit 100 has an output voltage that can easily be linearized with respect to firing angle. However, the full thyristor configuration also suffers from the disadvantage that it is costly due in part to the need to interface with both AC and DC breakers and the need to fire a de-excitation thyristor within the de-excitation circuit.
In order to provide a more cost-effective alternative to the full thyristor bridge 100, the hybrid bridge circuit 200, shown in FIG. 3, was developed. The hybrid bridge circuit 200 is suitable for applications in which response of the circuit is less critical than cost. Instead of the six thyristors 101-106 of the full thyristor bridge circuit 100, the hybrid bridge circus 200 includes three thyristors 101-103 and three diodes 151-153. A free wheeling diode 121 is connected across the output of the thyristors 101-103 and diodes 151-153. Replacing three of the thyristors with diodes reduces circuit cost. The hybrid bridge circuit 200 does provide the ability to de-excite the generator field 130, but has several undesirable effects. First, the hybrid bridge circuit 200 is subject to xe2x80x9clatch upxe2x80x9d where a rapid phase back can lead to a condition where the output voltage goes to about two thirds of ceiling or maximum voltage and will not recover with normal control action. Compensating for latch up requires extra control circuitry or firing circuit limits that don""t permit large firing angles. Restricting large firing angles limits the low voltage capability of the circuit and makes the circuit unusable if the ceiling voltage is high. Additionally, the hybrid bridge circuit output voltage as a function of firing angle deviates significantly from the full thyristor bridge circuit characteristic output voltage. Furthermore, the hybrid bridge circuit 200 generates line current harmonics that contain even harmonics. With the hybrid bridge circuit 200, current is maintained in the free wheeling diode 121 at a full load on the generator with normal ceiling voltages of 1.6 times generator full load voltage, a common specification. Furthermore, common mode voltage characteristics of a hybrid bridge are very different from those of a full thyristor bridge and have necessitated the use of special filter circuits to remove the effects of the common mode voltage.
In view of the deficiencies described above in the hybrid bridge circuit, a cost effective alternative to the full thyristor bridge circuit is needed that doesn""t have the deficiencies of the hybrid bridge circuit.
In accordance with the purpose of the invention as embodied and broadly described herein, there is provided a circuit for use in a synchronous generator excitation system, the circuit comprising: a three-phase bridge having three legs; means for connecting the three-phase bridge with an AC power supply; a thyristor in each of the three legs; and a de-excitation means connected in parallel with the three-phase bridge. The de-excitation means comprises a discharge path including free wheeling diode positioned to provide a discharge path for a field current.
In an additional embodiment of the invention, a circuit for use in a synchronous generator excitation system is provided. The circuit comprises a three-phase bridge having three legs and means for connecting the three-phase bridge with an AC power supply including a transformer. The circuit further comprises a thyristor in each of the three legs, wherein the thyristors provide a path for a device current. A de-excitation means is connected in parallel with the three-phase bridge. The de-excitation means comprises a discharge path including a free wheeling diode positioned to provide an automatic discharge path for a field current when a field voltage polarity reverses, wherein insubstantial current flows through the free wheeling diode until substantially all inductive energy of the transformer is depleted.
In yet an additional embodiment, the invention comprises a method for converting an AC voltage to a DC voltage using six thyristors forming a three-phase bridge and a free wheeling diode de-excitation mechanism connected in parallel with the three-phase bridge. The method comprises providing an AC power supply, triggering the thyristors when the AC power supply provides a positive voltage, and upon reversal of voltage polarity, automatically causing the free wheeling diode to conduct.
In yet an additional embodiment, the invention comprises a method for converting an AC voltage to a DC voltage using six thyristors forming a three-phase bridge and a free wheeling diode de-excitation mechanism connected in parallel with the three-phase bridge. The method comprises providing an AC power supply, triggering the thyristors when the AC power supply provides a positive voltage, and upon reversal of voltage polarity, depleting the inductive energy of a transformer associated with the power supply thereby causing a substantial portion of generator field current to discharge through the free wheeling diode, wherein after polarity reversal and prior to transformer discharge, an insubstantial current discharges through the free-wheeling diode.
The aforementioned embodiments and similar embodiments provide a low cost alternative to the full thyristor bridge circuit without all of the disadvantages of the hybrid bridge circuit. While the ability to transiently invert field voltage is not maintained by the apparatus of the invention, many other desirable characteristics of the full thyristor bridge circuit are maintained. The circuit of the invention is capable of reliable de-excitation and can be used with AC or DC breakers without the need for coordination with control. In contrast to the hybrid bridge circuit, the bridge circuit of the invention does not suffer from latch up and does not generate even line current harmonics. Line current harmonics at full load are the same as with the full thyristor bridge circuit. Furthermore, the bridge circuit of the invention has common mode voltage characteristics that are very similar to those of the full thyristor bridge circuit.
These and other features, objects, and advantages of the preferred embodiments will become apparent when the detailed description of the preferred embodiments is read in conjunction with the drawings attached hereto.